1. Field of the Invention
The invention relates to a process for inducing porosity in an abrasive articles by addition of a polymer resin which has lower elasticity, less moisture sensitivity, and improved thermal decomposition to the abrasive articles when forming. The invention further includes an unfired abrasive article comprising the polymer resin, and a pore inducer comprising the polymer resin.
2. Technology Review
Pores in an abrasive tool such as a grinding wheel are important. Pores, especially those which are interconnected in an abrasive tool, play a critical role in providing access to grinding fluids such as coolant to transfer the heat generated during grinding. In addition, pores supply clearance for material (e.g., metal chips) removed from an object being ground. These roles are particularly important in deep cut and modern precision grinding processes (i.e., creep feed grinding) for effectively grinding difficult-to-machine high performance alloys and hardened metals where a large amount of material is removed in one deep grinding pass without sacrificing the accuracy of the workpiece dimension. The porosity often determines the quality of the workpiece (such as metallurgical damage or "burn", and residual stresses), wheel life, cutting efficiency and the grinding power. Therefore, a high-porosity abrasive tool is often desired in many grinding applications.
Porosity is formed by both the natural spacing provided by the natural packing density of the materials and by conventional pore inducing media called "pore inducers" such as for example hollow glass beads, beads of plastic material or organic compounds, ground walnut shells, foamed glass particles and bubble alumina. While these conventional pore inducers provide porosity in the fired abrasive tool, there are drawbacks to their use. These drawbacks include one or more of the following: closed porosity, high springback, high moisture sensitivity, and incomplete thermal decomposition.
Springback is a measurement of the change in dimensions of an abrasive article over time after the release of pressure from molding or forming. The change in dimension of the abrasive tool is to a substantial extent affected by the elastic modulus of the material used as a pore inducer if the pore inducer is present in large enough quantities. Because of springback and its unpredictable nature, the accurate dimensions of a molded abrasive tool are often uncontrollable; therefore, the abrasive tool is off in its specification and properties making the process of producing the abrasive tools difficult to control.
Moisture absorption is the amount of water (H.sub.2 O) a pore inducer absorbs. High moisture absorption results in inconsistency in a pore inducer used in production of abrasive tools, and the change in water content affects the mixing, forming and firing of the abrasive tool. The humidity changes from day to day or season to season will change the water content of the final abrasive tool composition when a moisture sensitive pore inducer is used. Further, the variable moisture content makes the mixing, forming and firing of the abrasive tool more difficult. In addition, because of the unpredictability of the moisture content, the strength of the unfired wheels also become unpredictable.
Thermal decomposition behavior is the degree of decomposition of the pore inducer. Clean burn-off of the pore inducer below a certain temperature (such as glass transition point, T.sub.g, of the vitrified bond, .about.500.degree.-600.degree. C.) is desirable. Any residual pore inducer such as ash and/or charred carbon will result in a grinding wheel with "coring" problems, uncompletly induced pores and/or will result in changes in properties. Coring not only creates a "blackening" of the interior and at times the surface of the abrasive tool, it causes differences in properties and performance of the abrasive tool where the residual carbon due to its non-wetting nature with oxides can result in a weaker bond between the abrasive and the bond.
What is desired therefore is to provide a process of manufacturing abrasive tools with polymer resins having low moisture absorption which completely thermally decompose below the glass transition temperature of the vitrified bond, and when incorporated into the abrasive tool result in a tool with low springback and result in an abrasive article with properties similar to those made with conventional pore inducers.